By generating over 10 of millions metric tons of material, hundreds of billions of dollars of production per year, and being responsible for approximately millions of jobs, plastics and related businesses represent the fourth largest industry in the United States. Unlike other material industries such as steel and aluminum, however, this industry depends almost solely on nonrenewable raw material, mostly imported petroleum. This dependence becomes even more significant as the growth rate of plastics continues to outpace that of all other materials.
Most of the plastic supplied by today's manufacturers ends its life in landfills or incinerators simply because the technology has not been available to recover it economically. The Environmental Protection Agency estimates that the amount of plastic in municipal solid waste grew from less than 1 million metric tons prior to 1960 to over 20 million metric tons by 2000. Take-back and producer-responsibility legislation is becoming increasingly common to help deal with the quantities of plastics being produced.
Durable goods, such as automobiles, appliances and electronics equipment, account for about one-third of the plastics in municipal solid waste. Durable goods are increasingly being collected and partially recycled at the end of their useful lives to avoid disposal costs and potential liabilities, and to recover metals and other marketable raw materials.
The recovery of plastics from durable goods requires a plastic-rich raw material. Automobiles, appliances and electronics generally contain metals. Generally, the metals content is higher than the plastics content (typically plastics content is less than 30%) in these items, so a metal recovery operation must precede plastic recovery. Most metal recovery operations shred equipment in order to cost-effectively liberate metals from whole parts. Large-scale plastic recovery operations must be able to source this plastic-rich raw material from a number of metal recovery operations.
Most plastic parts coming from durable goods streams present unique challenges that are not met by the plastics bottle cleaning and sorting processes developed for curbside feedstocks. The principle practice today for the recovery of highly contaminated scrap is hand-separation done overseas at significant local environmental cost. The challenges in recycling plastics from durable goods include: multiple plastic types, multiple resin grades of plastic (there can be over 50 different grades of one plastic resin type, such as acrylonitrile butadiene styrene (ABS)); fillers, reinforcements, and pigments; metal; paint and metallic coatings; and highly variable part sizes and shapes.
A grade of plastic is a formulation of plastic material with a particular set of targeted physical characteristics or properties. The particular physical characteristics or properties of a grade are controlled by the chemical composition of the polymers in the grade, the average molecular weights and molecular weight distributions of polymers in the grade, the rubber morphology for impact modified grades, and the group of additives in the grade.
Different grades of a given plastic type are generally compatible. Grades can generally be melt mixed to create a new material with a different property profile. Different plastic types, on the other hand, cannot generally be melt combined as easily unless the types happen to be compatible. Blending different plastic types such as high-impact polystyrene (HIPS) and ABS is often avoided except in special situations.
Typical suppliers of plastics-rich feed stocks are metal recyclers or shredders who can process a number of types of durable goods in a single facility. Feedstocks derived from durable goods can therefore be highly variable mixtures of different types of durable goods. Generally, the plastics are broken down into particles which are typically less than about 100 mm in size, such as flakes or pellets. The particles can be formed from a single material or a combination of materials, such as various plastics, rubber, metal or other materials.
In order to create high value products, the plastic recycling process must be able to separate highly mixed streams on a flake-by-flake basis to achieve high throughput rates of products with acceptable purity. Methods such as separation by density, Density Differential Alteration, froth flotation, color sorting and triboelectrostatic separation (TES), have been used alone or in combination to achieve some purification of the plastics derived from durable goods, as described, for example, in, U.S. Pat. No. 6,238,579, U.S. Pat. No. 6,335,376, U.S. Pat. No. 5,653,867, U.S. Pat. No. 5,399,433, and U.S. Provisional Application No. 60/397,980, filed on Jul. 22, 2002, each of which is incorporated by reference herein. The acceptable purity depends on the primary plastic and contaminants.
TES is a low cost technique known for separating a simple plastic mixture because it has a negligible energy requirement and the device is rather simple. Because of its great potential for application in the separation and purification of plastics from durable goods, TES is one of the techniques that can be employed in plastics recycling plants.
TES is a relatively simple technique. Particles typically gain or lose electrons when they come into contact with other particles or with parts of the process equipment in a TES separator. Such charging by contact or friction is known as triboelectric charging, as described, for example, in W. R. Harper, Contact and Frictional Electrification, Oxford University Press, 1967.
In TES separation, the feed material is charged in a charging device. The charged material is then passed through a high-voltage electric field in the TES separator such that the material is deflected depending on its charge to mass ratio. As shown in TES separator 100 of FIG. 1, an electrostatic separation region 105 in TES separator 100 includes negative and positive electrodes 110, 115. As the plastic feed falls by gravity through separation region 105, positively charged plastic 120 deflects toward negative electrode 110 and the negatively charged plastic deflects toward the positive electrode. At the end of separation region 105, a baffle is arranged to deflect the original feed material stream into different collection containers. For example, a material with a zero initial velocity falling vertically through a horizontal electric field deflects horizontally by an amount proportional to the charge to mass ratio of the particle, as shown in Equation 1.
                              d          L                =                                            q              ⁢                                                          ⁢              E                                      m              ⁢                                                          ⁢              g                                ∝                      q            m                                              (        1        )            In practice, this deflection ranges from several centimeters to half a meter. The portion in the middle of the separation region, the middlings, generally has a charge that is between the materials that are deflected in either direction, or close to neutral.
One of the difficulties encountered in TES is the fact that there is a distribution of charge to mass ratios in any species of a mixture. This means that there will also be a distribution of particle deflections after falling through an electric field. The broader this distribution, the less likely it will be to separate different types of materials. In order to achieve a consistent separation with high purity products, it can be important to control the charge to mass ratios of particles fed to the TES separator, as described, for example, in International Application No. PCT/US03/11642, filed on Apr. 14, 2003, which is incorporated herein by reference. The distribution of charge to mass ratios that determines the distribution of particle deflections includes a contribution from a charge per surface area distribution and a contribution from a surface area to mass distribution. Controlling the charge to surface area and surface area to mass distributions leads to a more consistent and adequate separations. Techniques for controlling the surface area to mass ratio of mixtures fed to a TES separator developed by MBA Polymers are described in commonly-assigned U.S. Provisional Application No. 60/397,948, filed on Jul. 22, 2002, which is incorporated by reference herein.
Commonly-assigned U.S. Pat. No. 6,452,126 describes a technique known as “mediated triboelectrostatic separation” that enables more consistent and effective charging of plastics in mixtures as complex and variable as post-consumer plastics. Mediation controls the charge per surface area of the particles to be separated by adding an extra component known as “media” to the mixture. Media can include a single component or multiple components. The media is added to the mixture in such excess that the charge on the components to be separated is controlled solely by their ability to charge relative to the media.